Collaborative spatial multiplexing (CSM) is a mandatory feature in IEEE 802.16 (commercially known as WiMAX®) and the 3rd Generation Partnership Project Long Term Evolution (LTE) based communications systems. CSM is a bandwidth-saving technique, where two or more client stations (CSs) transmit their uplink data simultaneously using the same bandwidth resource. There is an apparent collision of the traffic between individual CSs when transmitting simultaneously using the same bandwidth resource and this normally would cause interference. However, by using antenna arrays at a base station (BS), the interference can be removed by using multiple-input multiple-output (MIMO) detection techniques, as well as carefully selecting the CSs involved in CSM uplink transmissions.
The performance of CSM depends on the spatial-dissimilarity between the spatial signatures of the CSs involved in uplink CSM. The difficulty in CSM scheduling for orthogonal frequency division multiple access (OFDMA) based technologies, such as WiMAX and LTE, is that the BS has limited knowledge of the spatial signatures of the CSs across the uplink spectrum since the spatial signature is frequency dependent, and CSs will likely be transmitting on different sets of subcarriers. Thus, the BS may not have the spatial signature of all CSs involved in a potential uplink CSM session in the frequency subcarriers that the BS would like to schedule for an upcoming CSM session. Hence, the BS cannot determine the degree of spatial signature correlation for uplink transmissions from the CSs.